The invention relates to a control circuit for controlling the solenoid driver of a dispenser that deposits fluid upon a conveyed substrate. More specifically, the invention relates to such a control circuit that compensates for the pull-in delay and drop-out delay inherent in the dispenser so that the dispenser deposits a bead of fluid commencing at a preselected position for a preselected duration.
In many phases of manufacturing there is a need to activate a responsive device which will act on a moving object. In the packaging or product assembly phases of manufacturing, for example it is often desired to apply a bead of adhesive of a given length to a specific area of an object (or substrate) while the substrate moves on a conveyor past a dispensing device. Generally, the dispenser must be turned on and off at precise times in order to apply the adhesive to the proper area on the object. For ease of understanding, the invention can be described in terms of this one specific application. Many other applications are of course possible.
In order to activate the dispenser in automated systems, a sensor is generally employed to detect the substrate moving on the conveyor. The sensor is generally located to sense the presence of the substrate upstream from the dispenser. Therefore, the activation of the responsive device must be delayed for some period of time after the substrate is sensed, specifically, until the substrate reaches the dispenser. Thereafter, the dispenser is activated for some given duration of time, during which adhesive is applied to the substrate.
The amount of time for which the start of the activating control signal must be delayed and the duration of the activating signal are influenced by many factors such as conveyor speed, distance from the sensor to the dispenser, the distance between the triggering edge of the object and the location on the object which the bead is to start (for turn on) or bead length (for turn off), and the time required for the dispenser to turn on in response to a control signal (hereinafter characterized as "pull-in delay") or drop out in response to removal of the control signal (hereinafter characterized as "drop-out delay"), or other system delays which are constant as a function of time irrespective of conveyor speed.
Each dispenser has an inherent pull-in delay and drop-out delay that is unique to itself. In applications using multiple dispensers that require particularly critical placement of fluid (e.g. hot melt adhesive), it is necessary that the particular delays of each dispenser be compensated for. Systems using a single time (delay-duration) have been unable to compensate for each dispenser. In order to compensate for each individual dispenser, the compensation (or control) circuit for the driver should be physically located at the solenoid driver. This type of compensation cannot easily be done with earlier devices.
In some applications the combination of such factors as dispensing duration and pull-in delay may be such (e.g. dispensing duration less than pull-in delay) that it is impossible for the dispenser to deposit the bead of adhesive in the correct fashion. Earlier devices have been unable to compensate for this sort of problem with the result being that a bead is not deposited.